Series capacitor protective device



JW@ 344 L.. R. LUDWMG ET AL.

SERIES CAPACITOR PROTECTIVE DEVICE Filed April 4, 1942 (bmi/"assed-WTNESSES:

ATTORNEY Patented June 20, 1944 SERIES CAPACITOR PROTECTIVE DEVICE LeonR. Ludwig, Forest Hills, and Charles V.

Fields, East Pittsburgh, Pa., uslgnors to Westlnghonse ElectricManufacturing Company, East Pittsburgh, Pa., a corporation ofPennsylvania Application April 4, 1942, Serial No. 437,636

9 Claims.

Our invention relates to protective devices for series capacitors suchas are connected in series with an alternating-current line orapparatus. While our invention is of more generic application to anykind of series-capacitor installation, our invention is particularlyadapted to the utilization of series capacitors in series with theindividual phase-conductors of polyphase electric power-transmittinglines connecting synchronous machines in such manner that stability orpower-limits are critical limitations, and where, for these reasons, itis particularly important to utilize high-speed switching on thetransmission line, for clearing any fault thereon, and to utilize acapacitor-protecting means which will very quickly restore the capacitorto service, after the clearing of the fault on the line. so that thecapacitor will be available to assist in maintaining system-stabilityduring the critical surge-period immediately following the clearing ofthe fault, when the system is particularly likely to surge itself out ofstep. The series capacitor synchronous line application just referred toconstitutes the subject-matter of a copending application of R. D.Evans, R. E. Marbury and A. C. Monteith, Serial No. 445,562, filed June3, 1942, assigned to the Westinghouse Electric i: Manufacturing Company.

Series capacitors, which are connected in series with analternating-current line or apparatus, need protection in mostinstances, particularly where the capacitor-voltage is high, and wherethe short-circuit current of the line or apparatus is more than two orthree times the normal full-load current. This is so, be-

cause the cost of the series capacitor increases exponentially inaccordance with the voltagelevel for which its insulation must bedesigned, and the voltage appearing across a series capacitor, varies,of course, with its current. It is usually economically necessary toprovide protective equipment forY limiting the amount of voltage whichcan appear across a series capacitor during fault-conditions, therebymaking it possible to design the capacitor at a voltagelevelcommensurate with the full-load current of the line or apparatus inwhich the capacitor is connected.

Arcing protective gaps have commonly been utilized, in shunt-circuitrelation to series capacitors, for arcing over, and by-passing orshortcircuiting the line-current from the series capacitor, in responseto excess-voltage conditions across the capacitor.` In suchprotective-arc devices, it is usually quite important for the arcingvoltage to be low, and for the recovery-voltage to be low, referring tothe voltage necessary to restrike the arc after each current-zero. Insuch arc-devices, there is obviously a problem connected with theextinguishment of the arc after the fault-current condition has passed.If magnetic blowout means were utilized, or even magnetically operatedarc-blast means, for extinguishing the arc in the protective gap, therewould be the danger, in some cases, of the magnetic force being sostrong, by reason of the combination of an excessive magnitude of thefault-current, plus the excessive magnitude of the surge-dischargecurrent of the series-capacitor bank discharging through the gap-device.that premature interruption of the protective arc might result, whichwould thereupon leave the capacitor in series with the line or apparatuswhile excessive currents are owing, in excess of the maximum currentwhich could be permitted to flow, with safety, through theseries-capacitor bank, even momentarily. If an air-blast were utilized,it might even be impossible for the protective gap to restrike, whilethe air-blast is playing, even though the applied voltage might be quiteexcessive. On the other hand, if the fault-current were of a lowervalue, there might be danger that the magnetic blowout, or themagnetically operated air-valve, might fail altogether to operate, thusleaving thearc playing continuously across the protective gap, andruining the gap which is strictly limited in its ampere-second rating.

In accordance with our present invention, we utilize a stable-arc deviceas the protective gapdevice, and we utilize the gap-current to start atiming-sequence controlled by high-speed relays whereby, after apredetermined time, an airblast is turned on, for blowing out the arc.In some instances, we may provide an undercurrent check-device formaking sure that the linecurrent has returned to normal full-load value,or less, before the air-blast is turned on. Preferably, also, we utilizesome sort of backup protection for closing a contactor-switch orcircuit-breaker in parallel-circuit relation across both theseries-capacitor and the protective-gap device in the event that thegap-current should not be interrupted by the airblast means withina timewhich is limited by the thermal capacity of the gap-device. Wepreferably also utilize means for quickly opening the contactar orcircuit-breaker upon the subsidence of the faultcurrent.

With the foregoing and other objects in view,

our invention consists in the apparatus, circuits, combinations, partsand methods hereinafter described and claimed, and illustrated in theaccompanying drawing, wherein Figs. 1 and 2 are diagrammatic views ofcircuits and apparatus illustrating the embodiment of our invention intwo diierent forms of embodiment.

In Fig. 1, we show our invention applied to the protection of a seriescapacitor 3 which is connected in series with a line 4-5, and which maybe isolated from the line by means of disconnectswitches 6. 1 and 8. Theseries capacitor 3 is shunted by a protective-gap device I0, whichpreferably has electrodes of tungsten or carbon or otherelectrode-material which would permit a stable are to form at the`proper overvoltage, the particular gap-device which we have illustratedbeing a carbon-electrode gap-device which has the property ofsublimizing, rather than liquefying, in response to great heat, so thatthe gap is not ruined by the passage of heavy currents for ten or morecycles, such a gap being described and claimed in the Marbury et al.Patent 2,144,503, granted January 17, 1939.

The gap-device II) may be arranged with such gap-spacing that it willbreak down, or arc over, at a predetermined over-voltage against whichit is desired to protect the series capacitor 3, or, as set forth in theMarbury Patent 2,072,717. granted March 2, 1937, it may be provided witha pilot-gap or trigger-gap II, which superimposes a high-frequency surgeon the main gap I0, causing the latter to break down when the pilot-gapbreaks down. The pilot-gap circuit includes an open-air inductance-coilI2 which is connected in series with the main gap III, and a capacitorI3 which is energized from an autotransformer I4, the pilot-gap II beingutilized to connect the inductance I2 and the capacitor I3 in parallelwith each other.

The main gap I may, or may not, be provided with a suitableimpedance-device for limiting the current-surge which is produced by theshort-circuiting of the main capacitor-bank 3 when the gap III breaksdown, and for this purpose I have illustrated the main gap-circuit asincluding a small surge-limiting resistor I5.

When the time comes to restore the seriescapacitor 3 to service, afterit has been by-passed by the gap-device I Il. we extinguish the arc inthe gap-device I0 by blowing a strong blast of air across itsarcing-space, which we accomplish by providing a nozzle I6, whichreceives air from a compressed-air tank I1, under the ycontrol of anormally closed v'alve I8, which is electrically operated by means of asolenoid I9, which is illustrated, in Fig. l, as being energized fromthe gapcurrent, as by means of a gap-circuit currenttransformer 2|. Ifthe compressed-air tank I1 is not too far away from the power-gap Il),and if the piping 22--23 from the tank to the nozzle I6 is fairlystraight, it will be possible to get the valve I3 open, and thecompressed air up through the piping to the nozzle IB and out across thearclng surfaces of the gap-device I0, within five to eight cycles, oreven less (on the basis of a 60cycle line), after the beginning ofcurrentow in the gap-circuit. l

This inherent time-delay, in the bringing of the air-blast into playacross the arcing terminals of the gap-device I0, may be relied upon asthe sole means for predetermining the length of time during which thearc will be permitted to play across the power-gap device I0, it beingunderstood that this time must be sufficiently long for thefault-responsive line-switching devices (not shown) to clear the faultfrom the line 4-5 before the arc is extinguished in the protectivegapdevice I0. If desired, however, the operatingtime can be adjusted toany desired predetermined value by means of any of the many knowntime-controlling means which are available, as symbolically indicated bymeans of a dashpot 24 in Fig. 1; vor undercurrent interlocking means maybe provided, as shown in Fig. 2, for making sure that the fault-currentconditions have passed, before permitting the air-blast to be applied.

In Fig. 2, we show a form of embodiment of our invention in which themain arc-device I0 is provided with an arc-chute 3D, which isdiagrammatically indicated in Fig. 2, and the air-blast is `utilized toblow the arc out of the arcing space between the two electrodesof themain gapdevice I0', and into an adjoining arc-interrupter or arc-chute30 where the arc may be easily quenched. l

In Fig. 2, we provide a gap-current relay GC which is energized inresponse to the current flowing in the gap-circuit I0', as by means of acurrent-transformer 2I', and we utilize this gapcurrent relay GC to setin operation a train of events which will result in the extinguishing oithe arc in the main gap-device I0'. In the illustrated form ofembodiment, in Fig. 2, the gap-current relay GC is utilized toimmediately energize an auxiliary relay R, through a resistance 32, andalso a timer T, from a suitable direct-current source which is indicatedby the terminals and The relay R thereupon picks up, and seals itselfin, through an auxiliary make-contact 33. 'I'he timer T begins tooperate, and will close its contact 34 in 6 cycles, or othertime-setting.

In Fig. 2, we also provide an overcurrent relay OC, which is energizedin accordance with the line-current, as by means of acurrent-transformer 35. The overcurrent relay OC picks up at the sametime as the gap-current relay GC, and instantly opens its threeovercurrent backcontacts 36, 31 and 33, getting these contacts openbefore the GC relay closes its single makecontact, which is suilicientlyidentified by reference to the GC designation. The GC contact, and theR-relay contact 33, in parallel, are utilized, in series with theovercurrent 'back-contact 31, to energize the solenoid-coil I9 of theair-valve I8 as soon as the current subsides, from fault-magnitude, to amagnitude very slightly above the full-load current, thereupon quicklyturning on the air-'blast and extinguishing the arc in the mainprotective device IU tby blowing the arc out into the arc-chute orinterrupter 30.

When the fault-current subsides and the overcurrent relay OC drops out,its `back-contact 38 short circuits the operating-coil of the auxiliaryrelay `R, thereby causing the latter to drop out, opening its holdingcontact 33, but not until the solenoid-operated valve I3 has beenopened.

If, however, the fault-current should persist, so that the air-blast isnot turned on within the time-setting of the timer T, which may be 6cycles, or other time determined by the thermal rating of the main-gapdevice I0', the timercontact 34 closes, and energizes the closing coilCCof a circuit-breaker 4I! 'which thereupon closes its main contacts 40which are connected in a shunting circuit around yboth theseries-capacitor 3 and the main arc-device IU. The total time of thetimer T and the time required for .the closing operation of the breaker40 must be within the thermal ampere-second rating of the maingap-device I' for the maximum faultcurrent magnitude which is to beexpected. 'I'he closing of the breaker-contacts 40 deenergizes the arcin the main gap-device l0', thus causing the arc to be extinguished andreleasing the gap-current relay GC, which thereupon drops out andreleases the auxiliary relay R and the timer T, resetting the same. Toprevent unnecessary overheating of the closing coil CC, the latter isusually connected in series with the 'back-contacts of a cutoff-relay Y,the operating coil of which is energized from an auxiliary switch 40awhich closes when the circuit-breaker 4D closes.

'I'he circuit-breaker 40 remains closed until the line-current subsidesto substantially full.- load value, whereupon the overcurrent relay OCdrops out and closes its back-contact 36, which energizes the trip-coilTC of the circuit-breaker 40, the tripping circuit being completedthrough an auxiliary breaker-switch 40a .which is closed when thebreaker is closed and which opens when the breaker opens.

The operation of both Figs. 1 and 2 may now be briefly reviewed. In bothfigures, the series-capacitor bank 3 remains shorted out of service aslong as the main arc-device I0 or l0' is arcing. and this time iscontrolled by the time required to get the air-blast across thearcing-space, so as to extinguish the arc. The carbon-electrodegap-device i0 or I0 has such a low arcing-voltage, and such a lowrecovery-voltage, that it not only constitutes practically ashort-circuit on the capacitor-bank 3, while the arc is in operation,'but the arc restrikes very promptly after each current-zero, as long asthe arc is not extinguishes by the turning on of the air-blast.

In Fig. 1, the arcing over of the main gap-device I0 initiates a flow ofcurrent in the gapcircuit, which is utilized to energize the electricsolenoid-valve Iii-I8. The nature of the piping 22--23 which is utilizedto feed compressed air to the nozzle I6, and the spacing of the nozzleI6 from the main-gap device I0, determine the time required to get theair-blast across the main gap-device I0, and insure that this time shallbe longer than the time during which current of fault-magnitude willever flow in the line 4 5 in which the series-capacitor 3 is connected.I'his predetermined time may be built directly in the apparatus, as juststated, or if some local or field-adjustment of the time is desirable, adashpot 24 may be provided, as indicated.

In Fig. 1, it is contemplated that some sort of auxiliary protectivecontactor or circuit-breaker 40' shall be provided, as isdiagrammatically indicated in the gure, the details of control of thisprotective contactor or circuit vbreaker 40' being omitted in Fig. 1,other than to indicate that the contacts are normally open, andconnected in shunt-circuit relation to both the main seriescapacitor-bank 3 and the main gap-device I0. In Fig. 1, this auxiliarycontactor or circuit-breaker 40' may be closed, either manually orotherwise, in the event of a defect in the apparatus or in the event ofa necessity for performing a service or maintenance-operation on theequipment.

In Fig. 2, instead of building a predetermined time-delay into theair-blast apparatus, so that the air-blast is automatically turned on,within such predetermined time-period, without waiting to check on thesubsidence of the fault-current,

we provide means for first checking, to be absolutely sure that thefault-current has subsided, before permitting the electricsolenoid-valve IS-IB to be energized, and thereafter, the time o!getting the air-blast across the electrode-surfaces of the maingap-device I0' is made as short y as can conveniently or economically bedevised,

- relay T. As long as current is flowing in the main gap-device I0', thegap-current relay GC remains actuated, energizing the timer T, whichrequires a certain predetermined time, which may be something like sixcycles, more or less, after which time the timer T closes its contacts.If the gap-arc is extinguished before the expiration of the time forwhich the timer T is set, the gapcurrent relay GC will drop out and thetimer T will reset itself for a new operation. If, however,

. the arc in the gap-device I0 is not extinguished by the expiration ofthe time for which the timer T is set, the timer-contact will directlyenergize the closing coil CC of the breaker 40 and bring about aclosing-operation of the breaker, which will be completed in some oneand one-half to four or more cycles after the closing of thetimercontacts 34.

In Fig. 2, also, we 4provide means for immediately and quickly reopeningthe circuit-breaker 40 as soon as the fault-current subsides, a functionwhich we accomplish by means of the Abackcontacts 36 of the overcurrentrelay OC, in series with the auxiliary breaker-switch 40a.

In Fig. 2, also, we have made a diagrammatic showing of a remote-controldevice, which we have indicated schematically by means of a twopositionswitch RC, having a make-contact 50, a break-contact 5|, and one or morehandles 52. When the automatic operation of the device, as heretoforedescribed, is in operation, the remotecontrol mechanism RC is in theillustrated position, with the closed back-contact 5I in series with thetripping circuit of the trip-coil TC. If it should be desired to by-passthe capacitor 3, as for the purpose of isolating it from the line 4 5,for repairs or servicing, the remote-control device may be actuated toits other position, as by means of the handle 52, whereupon thetripcircuit is opened at 5|, so that the breaker will not beautomatically tripped, and the make-contact 50 of the remote-controlswitch RC will be closed to directly energize the closing-coil CC of thecircuit-breaker 40. After the breaker 40 has been closed, it will besafe to close the disconnectswitch 6 which by-passes the entireseries-capacitor equipment, after which the isolatingdisconnecting-switches 1 and 8 may be safely opened.

While we have illustrated our invention in two preferred forms ofembodiment, we desire it to be understood that our invention,particularly in its broader aspects, is susceptible of embodiment inother forms of embodiment, with various substitutions, omissions andchanges, such as will be obvious to those skilled in the art. We desire,therefore, that the appended claims shall be accorded the broadestconstruction consistent with their language and the prior art.

We claim as our invention:

l. A series-capacitor installation for an alternating-current line,comprising, in combination with said series capacitor, a protectivegap-device connected in shunt-circuit relation to said series capacitor,means including an electrically controlled valve for supplying a blastof gas to said gap-device, and means responsive to an excessivecurrent-iiow through said gap-device for causing the valve to open at atime subsequent to the subsidence of the line-current. l

2. A series-capacitor installation for an alterhating-current line,comprising, in combination with said series capacitor, a protectivegap-device connected in shunt-circuit relation to said series capacitor,means including an electrically controlled valve for supplying a blastof gas to said gap-device, and means responsive to current-now throughsaid gap-device and to a subsidence of the line-current to apredetermined value for causing the valve to open.

3. A series-capacitor installation for an alternating-current line,comprising, in combination with said series capacitor, a protectivegap-device connected in shunt-circuit relation to said series capacitor.means including an electrically controlled valve for supplying a blastof gas to said gap-device, and time-delay means responsive to anexcessive current-flow through said gap-device for causing the valve toopen, the time-delay interposed by said time-delay means beingsuilicient to allow a reasonable time for the subsidence of theline-current.

4. A series-capacitor installation comprising, in combination with saidseries capacitor, a protective gap-device connected in shunt-circuitrelation to said series capacitor, means including an electricallycontrolled valve for supplying a blast of gas to said gap-device, anormally substantially open-circuit auxiliary bypass-means inshunt-circuit relation to both the series capacitor and the protectivegap-device. and means responsive to current-now through said gap-devicefor initiating a train of events resulting first in an opening operationof the valve under certain circumstances and resulting subsequently inthe establishment of a closed-circuit condition of said auxiliarybypass-means in the event of an imperfect operation of theprotective-gap and gasblast apparatus.

5. A series-capacitor installation comprising, in combination with saidseries capacitor, a protective gap-device connected in shunt-circuitrelai tion to said series capacitor, means including an electricallycontrolled valve for supplying a blast of gas to said gap-device, anormally substantially open-circuit auxiliary bypass-means in shuntlcircuit relation to both the series capacitor and the protectivegap-device, means responsive to current-flow through said gap-device andto a subsidence of the line-current to a predetermined value for causingthe valve to open, means responsive to current-flow through saidgap-device for a predetermined time for establishing a closed-circuitcondition of said auxiliary bypassmeans, and means responsive to thelast-named event and to a subsidence oi' the line-current to apredetermined value for substantially restoring the normal substantiallyopen-circuit condition of the auxiliary bypass-means.

6. A series-capacitor installation comprising, in combination with saidseries capacitor, a protective gap-device connected in shunt-circuitrelation to said series capacitor, means including an electrically'controlled valve for supplying a blast o! gas to said gap-device, amanually and automatically controllable circuit-interrupter means inshunt-circuit relation to both the seriescapacitor and the protectivegap-device, means responsive to current-flow through said gap-devicei'or causing the valve to open under certain conditions, to clear thearc from the gap-device, means responsive to a failure of thegap-clearing means to operate within a predetermined time for causing acircuit-making operation or said circuit-interrupter means, and meansresponsive to the last-named event for causing a circuitinterruptingoperation of the circuit-interrupter means under certain conditions.

7. A series-capacitor installation comprising. in combination with saidseries capacitor, a protective gap-device connected in shunt-circuitrelation to said series capacitor, means including an electricallycontrolled valve for supplying a blast of gas to said gap-device, andmeans responsive to current-ilow through said gap-device for initiatinga train of events resulting first in an opening operation of the valveunder certain circumstances and means further responsive to current-nowthrough the gap-device for producing an effect resultingsubsequentlyilin closing a gap-shunting circuit for causing the seriescapacitor to be by-passed by a gap-free all-conductor circuit in theevent of an imperfect operation of the protective-gap and gas-blastapparatus.

8. A series-capacitor installation comprising, in combination with saidseries capacitor, a protective gap-device connected in shunt-circuitrelation to said series capacitor, means including an electricallycontrolled valve for supplying a blast of gas to said gap-device, meansresponsive to current-dow through said gap-device and to a subsidence ofthe line-current to a predetermined value for causing the valve to open,timedelay circuit-means operative after the establishment of current-nowthrough said gap-device for effecting a circuit-closing operation forcausing the series capacitor to be by-passed by a gap-free all-conductorcircuit in the event of an imperfect operation of the protective-gap andgas-blast apparatus, and means responsive to the last-named event and toa subsidence of the line-current to a predetermined value for performinga circuitopening operation restoring the apparatus to its initialcondition.

9. A series-capacitor installation comprising, in combination with saidseries capacitor, a protective gap-device connected in shunt-circuitrelation to said series capacitor, means including an electrically`controlled valve for supplying a blast of gas to said gap-device, amanually and automatically controllable circuit-closing means, meansresponsive substantially to an initiation of current-ilow through saidgap-device for causing the valve to open under certain conditions, toclear the arc from the gap-device. means responsive toa failure of thegap-clearing means to operate within a predetermined time for causing acircuit-making operation of said circuit-closing means in such manner asto cause the series capacitor to be by-passed by a gap-freeall-conductor circuit, and means responsive to the lastnamed event forcausing a circuit-interrupting operation of the circuit-closing meansunder certain conditions.

LEON R. LUDWIG. CHARLES V. FIELDS.

